1. Field of the Invention
This invention relates to a cube root calculation apparatus, namely to an apparatus for obtaining the cube root of an arbitrary number.
2. Description of the Prior Art
Generally, the cube root of an arbitrary number is obtained by either the method using a logarithmic function and exponential function and the method using Newton's successive-approximation equation.
In the former, the logarithm of a number from which the cube root is to be calculated (hereinafter, such a number is referred to as "a cube root extraction number") is obtained, and the exponent of the number obtained by dividing the logarithm by three is calculated.
In the latter, Newton's successive-approximation equation is applied to a cube root extraction number "A": EQU X.sub.n+1 =2.multidot.X.sub.n /3+A/X.sub.n.sup.2 (n=0, 1, 2, . . . )
Then, the values X.sub.1, X.sub.2, X.sub.3, . . . are successively calculated. When X.sub.m and X.sub.m+1 are equal to or approximately equal to each other, X.sub.m is determined as the cube root of the cube root extraction number "A".
The above-described methods of the prior art require the calculation of special functions such as logarithmic and exponential functions, and also repeated multiplications and divisions, with the result that the time required for the calculation of the cube root of a number in the prior art becomes extremely long. Moreover, the number of digits of the mantissa of a cube root obtained in the prior art depends on the computational accuracy of subroutines used to execute the calculation of the special functions, and also on that of the multications and divisions, so that the number of digits of the mantissa of a cube root cannot be arbitrarily specified.